Friction welding is the process for welding together two bodies or workpieces by converting mechanical energy to heat energy by the friction between the engaging weld surfaces of the two workpieces. In rotary friction welding, the process involves effecting relative rotation between the two workpieces while the weld surfaces remain in engagement with each other. In linear friction welding, the process involves effecting relative lateral oscillation between the two workpieces while the weld surfaces remain in engagement with each other.
Compressors and turbines of gas turbine engines such as those used in aero engines typically include a plurality of rotor and stator vane assemblies. The rotor assemblies are designed to impart work into gases passing through the compressor and extract work from the gases passing through the turbine. The stator vane assemblies help to direct the working gases entering or exiting the rotor assemblies and thereby increase efficiency of the engine.
Each rotor assembly includes a disk and a plurality of blades attached to the disk so as to extend radially outwardly therefrom. Conventionally, the blades are attached to the disk by mechanical connections such as “fir tree” type connections where a fir tree shaped blade root is received within a complementary shaped recess in the disk. This means that the blade can be readily replaced in the event that it is damaged.
Recent developments have resulted in integrally bladed rotor assemblies or “blisks” in which the blades are formed integrally with the disk. These have advantages of reduced weight as compared to a standard rotor assemblies and improved aerodynamic efficiency.
Blisks can be formed by joining the blades onto the rim of the disk at respective blade stubs by linear friction welding (LFW). This is a process whereby one part is held stationary while the other part is oscillated against it under a forge load, the heat generated and the applied loads result in a weld, as upset material exudes as flash from the edges of the joint. Typically, the blade is oscillated relative to the disk, while the forge load is applied in the radial direction on the blade towards the disk or on the disk towards the blade. The blade is thereby joined to the disk.
An LFW machine has respective toolings at which the disk and the blade are mountable. To accommodate the forge load, the machine is relatively massive, and access to the relatively small working volume of the machine is restricted. However, it is essential that the blade and disk are suitable aligned at the start of the LFW process to ensure that the blade is welded to the disk at the correct position and in the correct orientation. To achieve this it is conventional to use a sacrificial set-up disk and blades matching the shapes of the production disk and blades. The set-up disk and a sacrificial blade are mounted in the respective machine toolings and welded together. Based on the outcome of that procedure, typically as determined by analysis of the dismounted welded blade and set-up disk using a coordinate measuring machine (CMM), adjustments can be made to the relative starting positions and orientations of the toolings. Further blades can be welded to the set-up disk as needed in order to refine the adjustments.
A disadvantage of this approach, however, is that it is time-consuming and also requires the production and consumption of relatively expensive sacrificial components.